Acousto-optic underwater detector

ABSTRACT

A low frequency or pressure-gradient hydrophone comprising an optical reflector experiencing displacements responsive to acoustic waves. A beam light from a light source is carried by a first group of fiber optics guides and is incident upon the optical reflector. The light reflected from the reflector is carried by a second group of fiber optics guides to a light detector. Any displacements of the reflector due to pressure gradient due to acoustic waves impinging on the opposite sides of the reflector are detected by changes in intensity of reflected light from the light source.

Quonne tee tent 1191 Aug. 20, 1974 1 ACOUSTO-OPTIC UNDERWATER DETECTOR[75] Inventor: Frank W. Cuomo, East Providence,

[73] Assignee: The United States of America as represented by theSecretary 01 the Navy, Washington, DC.

22 Filed: Apr. 14,1972

21 App]. 110.; 246,092

52 us. (:1 340/8 R, 73/71. 1, 250/227,

, 340/13, 350/96 B 51 1m. (:1 ..G01v l/16 58 Field of Search 350/96,193; 181/.5 NP,

[56] References Cited UNITED STATES PATENTS 3,533,056 10/1970 Clark340/8 X 3,541,848 11/1970 Thurstone 73/67.5

3,580,082 5/1971 Strack 73/7l.3 3,584,227 6/1971 Matsas 3,642,090 2/1972Bennett 181/.5 H

Primary Examiner-Benjamin A. Borchelt Assistant ExaminerH. J. TudorAttorney, Agent, or Firm-Richard S. Sciascia; Arthur A. McGill; PrithviC. Lall [57] ABSTRACT A low frequency or pressure-gradient hydrophonecomprising an optical reflector experiencing displacements responsive toacoustic waves. A beam lightfrom a light source is carried by a firstgroup of fiber optics guides and is incident upon the optical reflector.The light reflected from the reflector is carried by a second group offiber optics guides to a light detector. Any displacements of thereflector due to pressure gradient due to acoustic waves impinging onthe opposite sides of the reflector are detected by changes in intensityof reflected light from the light source.

4 Claims, 4 Drawing Figures I ACOUSTO-OPT 1C UNDERWATER DETECTORSTATEMENT OF GOVERNMENT INTEREST BACKGROUND OF THE INVENTION The presentinvention relates to underwater communication and particularly pertainsto low frequency hydrophones wherein any displacements of an opticalreflector resulting from pressure gradient on the opposite sides of thereflector because of impinging acoustic waves are detected by changes inintensity of light reflected by the reflector.

Transducers used in underwater communication for detecting acousticwaves generated by various sources usually employ piezoelectric crystalsof materials such as barium titanate, lead zirconate as acousticelements. However, such transducers are not very efficient at lowfrequencies, e.g., a few hundred hertz, or below. This is because ofrelatively high resonant frequencies of piezoelectric crystals used andtheir relatively low detec tion efficiency at these frequencies.Furthermore, they display omnidirectional patterns and have sizes of 12inches or longer in order to have receiving responses averaging 90decibels (abbreviated as dB) for generating signals of 1 volt permicrobar pressure of acoustic waves. Besides, these transducers do nothave a uniform low frequency response and different piezoelectriccrystals need be used to cover low frequency region. Consequently, theiromnidirectional characteristics, relatively large size, and inefficientoperation at low frequencies impair their usefulness in some underwateracoustic detection systems with particular reference to torpedoapplications.

SUMMARY OF THE INVENTION The objects and advantages of the presentinvention are accomplished by utilizing an acousto-optic transducerwhich comprises a bifurcated fiber-optic bundle housed in a light-tightbox, a lightweight optical reflector which is flexibly attached to thelight-tight box by means of a plurality of springs, a source of light,and a light detector. Light from the light source is carried through thebox by the first group of the bifurcated fiber optics bundle to thereflector. The second group of the bifurcated fiber optics bundlecarries the reflected light through the box to the light detector, thelight-tight box preventing any extraneous light from interfering.Pressure exerted by an incoming acoustic wave displaces the opticalreflector with resulting displacement of the reflector beingproportional to the pressure differential on the opposite sides of thereflector and thus varies the intensity of the light reflected from thereflector. By changing the springs used for attaching the reflector tothe box, it is possible to change the resonant frequency of the detectorand thus the frequency band of the transducer for a uniform response.The frequency response of the transducer can also be varied by usingoptical reflectors of different masses.

An object of this invention is to have an acoustooptic transducer havinga uniform low frequency response.

Another object of this invention is to have an acousto-optic transducerwhich has a directionality characteristic instead of beingomnidirectional.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically anacousto-optic transducer of subject invention;

FIG. 2 illustrates a partially exploded perspective view of thetransducer;

FIG. 3 illustrates a cross-sectional view of the transducer taken alongthe line 33 of FIG. 2; and

FIG. 4 illustrates a cross sectional view of the transducer taken alongthe line 4-4 of FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring more specifically to thedrawings, FIG. 1 illustrates an acousto-optic transducer 10 whichcomprises a light-tight box 12, a bifurcated fiber optics bundle 14having branches 16 and 18, a light source 20, an optical detector 22, anoptical reflector 24 attached to box 12 by means of spring biasedpistons 26 and 28 schematically shown in FIG. I. and in a perspectiveview in FIG. 2. Bifurcated bundle comprises a multiplicity of opticalfibers as shown in FIG. 4 which is a cross sectional view of FIG. 2along line 4-4. The optical fibers as shown in FIG. 4 are arranged insuch a way that each transmitting fiber which transmits light from lightsource 20 to the optical reflector 24 is completely surrounded byreceiving fibers which carry the light reflected by the opticalreflector 24 to detector 22 for increasing the efficiency of thetransducer. Cable 30 shown in FIG. 2 provides power to light source 20and detector 22.

FIG. 3 shows a cross sectional view along line 3-3 of the transducershown in FIG. 2. Two generally identical pistons 26 and 28 are springbiased and mounted in holes 32 and 34 respectively in wall 36 of box 12and cover plate 37 which is attached to box 12 by set of screws notshown in the drawings. Piston 26 comprises a wider section 38 fittingtightly in hole 32, a narrower section 40, and a threaded section 42.Likewise piston 28 comprises a wider section 44, a narrower section 46,and a threaded section 48. Springs 50 and 52 are inserted in holes 32and 34 respectively. Spring 54 is slipped on section 40 of piston 26abutting common shoulder 56 of sections 38 and 40. Likewise spring 58 isslipped on section 46 of piston 28 abutting common shoulder 60 ofsections 44 and 46. Pistons 26 and 28 are then inserted into holes 32and 34 respectively, the end of piston 26 abutting spring 50 and the endof piston 28 abutting spring 52. Reflector 24 is supported by a backplate 64 attached thereto all around except a small area 66 of thereflector 24. Back plate 64 is attached to the threaded sections 42 and48 of pistons 26 and 28 respectively by means of nuts 68, 70 and 72, 74.0 rings 76 and 78 are used to protect fibers 14 from water. Thereflector 24 is light and is generally made of materials such as glass,brass, and mylar coated with a reflector paint. However, it should beunderstood any reflector which is lightweight and having a highreflection coefficient can be used regardless of the material from whichit is made. By adjusting the position of the back plate 64 of theoptical reflector 24 or by changing springs 50, 52, 54, and 58, it ispossible to change resonant frequency of the transducer and therebychange frequency band for a uniform low frequency response of thetransducer. Furthermore, by using reflectors of 5 different masses, itis also possible to change frequency characteristics of the transducer.Replacement of the springs and the reflectors can be achieved withoutmaking any major modifications in the transducer. The transducer can beeither free flooding by making water flow on both sides of the reflector24 or by making water flow only one side of the reflector 24 byadjusting the spacing between box 12 and reflector 24.

Thus, an optical reflector which is adjustably attached to a light boxhaving a bundle of fiber optics transmitters and receivers passingtherethrough is displaced by an acoustic wave resulting from thepressure differential on the opposite sides of the reflector. Thisdisplacement of the optical reflector causes changes in intensity oflight reflected by the optical reflector and carried by fiber opticsreceivers to a light detector. Due to flexibility in the choice of massof the optical reflectors used, it is possible to change low frequencyresponse of the transducer.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. A pressure-gradient hydrophone comprising:

a light-tight box having a first side and a second side opposite saidfirst side;

a cover plate attached to said second side;

a source of light secured to said box;

a light detector secured to said box;

a bifurcated fiber optics bundle comprising a first bifurcated branchhaving a first multiplicity of fiber optics guides and a secondbifurcated branch having a second multiplicity of fiber optics guides,an end of said first bifurcated branch being in communication with saidsource of light and an end of second bifurcated branch being incommunication with said light detector, said first multiplicity of fiberoptics guides of said first branch and said second multiplicity of fiberoptics guides being intermingled at the opposite ends of said first andsecond bifurcated branches proximate said cover plate and passingtherethrough; and

an optical reflector having opposite sides, said reflector beingadjustably attached to the side of said cover plate distal saidbifurcated bundles, said sides being disposed to allow an incidentacoustic wave to impinge thereon;

whereby light is transmitted from said source of light through saidfirst bifurcated branch and reflected by said optical reflector,intensity of light reflected by said reflector being proportional todisplacement of said reflector because of pressure gradient across theboth sides of said reflector due to said incident acoustic waveimpinging thereon, then carried by said second bifurcated branch anddetected by said detector.

2. The hydrophone of claim 1 wherein said reflector is attached to saidbox by a plurality of pistons, said plurality of pistons being securedto said optical reflector on one side thereof and to said cover plate onsaid second side of said box;

3. The hydrophone of claim 2 wherein said plurality of pistons arespring biased at ends connected to said cover plate.

4. The hydrophone of claim 3 wherein said bifurcated fiber optics bundleis attached to said cover plate, passing therethrough, and having itsend facing said reflector.

1. A pressure-gradient hydrophone comprising: a light-tight box having afirst side and a second side opposite said first side; a cover plateattached to said second side; a source of light secured to said box; alight detector secured to said box; a bifurcated fiber optics bundlecomprising a first bifurcated branch having a first multiplicity offiber optics guides and a second bifurcated branch having a secondmultiplicity of fiber optics guides, an end of said first bifurcatedbranch being in communication with said source of light and an end ofsecond bifurcated branch being in communication with said lightdetector, said first multiplicity of fiber optics guides of said firstbranch and said second multiplicity of fiber optics guides beingintermingled at the opposite ends of said first and second bifurcatedbranches proximate said cover plate and passing therethrough; and anoptical reflector having opposite sides, said reflector being adjustablyattached to the side of said cover plate distal said bifurcated bundles,said sides being disposed to allow an incident acoustic wave to impingethereon; whereby light is transmitted from said source of light throughsaid first bifurcated branch and reflected by said optical reflector,intensity of light reflected by said reflector being proportional todisplacement of said reflector because of pressure gradient across theboth sides of said reflector due to said incident acoustic waveimpinging thereon, then carried by said second bifurcated branch anddetected by said detector.
 2. The hydrophone of claim 1 wherein saidreflector is attached to said box by a plurality of pistons, saidplurality of pistons being secured to said optical reflector on one sidethereof and to said cover plate on said second side of said box;
 3. Thehydrophone of claim 2 wherein said plurality of pistons are springbiased at ends connected to said cover plate.
 4. The hydrophone of claim3 wherein said bifurcated fiber optics bundle is attached to said coverplate, passing therethrough, and having its end facing said reflector.